A "point micrometer" is a special measuring tool that measures the thickness of an uneven surface. Here, it is used to measure the thickness of the brake track:
While such measurement is useful, note that it does not measure the inside the box section, which may be much thinner. In this case, the thinnest part in the box is about 0.35 mm, but the micrometer measures about 0.70 mm.
In addition the point micrometer cannot reach well in to corners very well. Here, it gets close, but hits the tube bed and cannot quite measure the thinnest part even outside of the box. Although it gets close here, a rim with a flatter tube bed would make it harder for the micrometer to reach in to the corner.
The point micrometer is also cumbersome to use because the tire and tube must be removed in order to measure rim wear. Since flat tires are often discovered while riding, this means fewer opportunities to measure with a point micrometer when tire and tube are off anyway.
Thus, the point micrometer is a good tool, but it would be better if makers provided a rim profile suitable for external measurement. Except in cases of uneven rim wear, that would probably give good accuracy without removing tire and tube, and would allow measurement of the region inside the box.
Point micrometers are specialized and tend to be expensive. A cheaper and less-accurate — but, probably, accurate-enough — alternative is a dental caliper (via http://bike-works.blogspot.com/2009/01/bike-rims-wear-out.html and http://www.yates-motloid.com, as of 2014/10):
As with the point micrometer, the dental caliper requires dismounting the tire, and cannot measure inside the "box"; in the rim photo above, the thinnest part is inside the box.